Water Vapour, Humidity, Condensation and Stability
Water vapour enters the air by evaporation and leaves it by condensation; the maximum mass it can hold rises steeply with temperature (roughly doubling for every 10 °C). Candidates must distinguish mixing ratio and specific humidity (mass-based, little changed by compression) from relative humidity (ratio of actual to saturation vapour pressure, changes with temperature) and from dew point and frost point.
Dew point and saturation
The spread between temperature and dew point indicates how close the air is to saturation. When the spread is small, condensation, fog or low cloud is likely. Condensation requires both saturation (cooling to dew point or adding moisture) and condensation nuclei. The dew point depression is a standard question hook: a small depression with a small diurnal temperature fall gives radiation fog.
Stability
Stability is determined by comparing the environmental lapse rate with the adiabatic rates. Absolutely stable: ELR less than SALR. Conditionally unstable: ELR between SALR and DALR. Absolutely unstable: ELR greater than DALR. Stability is enhanced by cooling the upper air, warming the surface, or adding moisture low down. Inversions are extremely stable and trap pollutants and moisture beneath them.
Worked example: ELR 2.5 °C/1000 ft with SALR 1.8 and DALR 3.0 means the ELR lies between them: conditionally unstable, so saturated air rises freely while dry air resists.
Trap: relative humidity can reach 100% without any cloud if no nuclei are present.